A.    An adjective clause is a group of words that gives more information about a noun.  In an adjective clause there is a marker (or relative pronoun), a subject, and a verb.

Examples:
The mountain that I see is covered with snow.
(“that I see” tells us which mountain)
The hospital where I  was born closed down.
(“where I was born” tells us which hospital)

B.  Sometimes the markers who, that, and which are also the subjects.

Examples:
The man who sat next to me was very old. (who is the subject of sat)
The color that goes best with green is grey. (that is the subject of goes)

C.  The adjective clause markers are:

(The adjective clause is in bold print.)

WHO  – used for people:

Examples:
The doctor who set my broken arm has retired.
I can’t remember the name of the person who helped us at the airport.

WHOM  – used for people:

Examples:
She won’t tell me the name of the man whom she called.
William, whom she loves deeply, gave her a diamond ring.

WHOSE  – used for possessions (things that belong to someone or something)

Examples:
The parents whose daughter had won the award stood up and cheered.
The next building whose exterior needed painting was city hall.

WHICH  – used for things or for an entire phrase:

Examples:
The fitness club which they just joined is close to their home.
The clown slipped on the banana peel, which made everyone laugh.

THAT  – used for people or things:

Examples:
The person that lives in that house is my best friend.
The furniture that we bought  will arrive tomorrow afternoon.

WHERE  – used for places or situations:

Examples:
The town where he was born is located in the Alps.
That’s the scene where the hero dies.

WHEN  – used for times:

Examples:
Do you remember the time when we got lost downtown?
The weekend when we went to Las Vegas was really exciting.

NOTE:  You can also use that for times.

Examples:
Do you remember the time that we got lost downtown?
The weekend that we went to Las Vegas was really exciting.

WHY  – used only after “the reason”:

Examples:
The reason why I was late was because my car had a flat tire.
I don’t know the reason why she hates me so much.

NOTE:  You can also use that after “the reason.”

Examples:
The reason that I was late was because my car had a flat tire.
I don’t know the reason that she hates me so much.

D.  You can drop who (whom,) that, which, when, and why (but never whose) when they are followed by a subject and a verb.

Examples:
I know the person who she lives with.
I know the person whom she lives with.
I know the person she lives with.

The book that he lost was his geometry textbook.
The book he lost was his geometry textbook.

I don’t agree with the choice which she made.
I don’t agree with the choice she made.

Sunday is the day when it‘s most likely to rain.
Sunday is the day it‘s most likely to rain.

The reason why I can’t stay is simple.
The reason I can’t stay is simple.

E.  Adjective clauses go right after the noun they describe:

Examples:
I once knew a man who could walk on his hands.
Vancouver, which has a growing population, is B.C.’s largest city.

Exception:    If there is a short prepositional phrase or a time after the noun, then the adjective clause goes after it.

Examples:
I didn’t see anyone in danger that I could help.
The man over there, whose name I don’t remember, is a famous artist.
The party last Saturday night, which was a lot of fun, ended at 3:00.
My history class this term, which I absolutely love, has been canceled.

Be careful.  Sometimes it seems like an adjective clause can describe two different nouns.
When this happens, place the adjective clause after the more general noun.

Examples:
An axe is a tool that you use to cut down a tree.
(NOT:  An axe that you use to cut down a tree is a tool.)
(Tool is more general than axe, so the adjective clause goes after tool.)

The day that I saw her was last Monday.
(NOT:  The day was last Monday that I saw her.)
(Day is more general than Monday, so the adjective clause goes after day.)

F.  There are two general types of adjective clauses.  One type has necessary information and the other type has extra information.

To know if a clause has necessary or extra information, ask yourself:
Do I know what the noun is without this information?
If the answer is yes, then the adjective clause has extra information and takes commas before and after it.
Also, if the adjective clause has extra information, then you can drop it, and the sentence is still understandable.

Examples:
My brother, whom I haven’t seen in years, is coming to visit.
I know who my brother is and don’t need the information “whom I haven’t seen in years.”  Also, I can drop the adjective clause (My brother is coming to visit.), and the sentence is understandable.
This information is extra and therefore has commas.

Vancouver, which lies on the west coast of Canada, is a major Canadian port.
[I know what Vancouver is and don’t need the information which lies on the west coast.]
Also, I can drop the adjective clause (Vancouver is a major Canadian port.), and the sentence is understandable.

Note:
If the noun is a proper noun (the name of someone or something starting with a CAPITAL letter) like Vancouver, the adjective clause will always be extra information and will have commas.

But:
The person that I want to see isn’t in his office.
That I want to see is necessary information because I don’t know who the person is without it.  Also, I cannot drop the adjective clause (The person isn’t in his office) because now I don’t know who the person is.  I need the information that I want to see.  Therefore, there are no commas.

G.  When you have an extra information adjective clause (with commas), you cannot use that.  You must use which for things and who/whom for people.

Examples:
I got my first bicycle, which was red, when I was six years old.
(NOT: I got my first bicycle, that was red, when I was six years old.)
His father, who was a jazz musician, lived into his nineties.
(NOT:  His father, that was a jazz musician, lived into his nineties.)
Jonathan, whom I met at a party last fall, is now my roommate.
(NOT:  Jonathan, that I met at a party last fall, is now my roommate.)

H.  There is a difference between who and whom, which is easy to remember.   Who is used before a verb, and whom is used before a subject and verb.  However, most English speakers don’t use whom but use who or that instead.

Examples:
The doctor who set my broken arm has retired.
She won’t tell me the name of the person whom she called.
Most people write:   She won’t tell me the name of the person who she called.
or:        She won’t tell me the name of the person that she called.
My brother, whom I haven’t seen in years, is coming to visit.
Most people write: My brother, who I haven’t seen in years, is coming to visit

I.  Which can also be used to modify a whole sentence.  In this case the adjective clause is extra information and must have a comma before it.